Because in most of the derivations of the Fock equation it's written [tex]dL[{\Psi_i}] = 0[/tex] But when we are considering an infinitesimal change on our wavefunction it can be on any of its variables: [tex]\frac{\partial L[{\Psi_i}]}{\partial \Psi_i}\frac{\partial \Psi_i}{\partial r_i}\partial r_i[/tex]